1. Field of the Invention
The present invention relates to a magnetic recording medium and a magnetic memory apparatus using the same, and, more particularly, to a pattern type magnetic recording medium having multilayered nanostructures having a magnetoresistance effect laid out discretely and a magnetic memory apparatus which writes and reads information from the recording medium using a cantilever array.
2. Description of the Related Art
The surface recording density of magnetic disk apparatuses is ever increasing and is expected to reach 100 gigabits per inch in 2003. The present in-plane recording system, however, has a problem that as the bit length to write becomes shorter, magnetic signals written on a recording medium are vanished by thermal fluctuation, which stands in the way of increasing the surface recording density. Attention is paid to a perpendicular recording system which writes magnetic signals in a direction perpendicular to the recording medium as a solution to this problem. In particular, as the perpendicular recording system which uses a perpendicular recording medium having a soft-magnetic backing layer as a recording medium and uses a single-pole head to write information has a high resistance to a thermal fluctuation and can generate a strong recording magnetic field, the perpendicular recording system is a promising future perpendicular recording system for super high density.
For example, FD-08 in the 8th Joint MMM-Intermag Conference held at San Antonio in America in January 2001 has reported perpendicular recording with a surface recording density of 60 gigabits per square inch (see Non-patent Document 1).
In the perpendicular recording system, as recording bits become smaller as the surface recording density increases, the area of the floating surface of the single-pole head to be used in writing information which faces a magnetic recording medium should be made smaller. As the area of the floating surface of the single-pole head is reduced, however, the intensity of the magnetic field that can be generated is reduced in inversely proportional to that area. This results in insufficient recording. A first solution to this shortcoming is to increase the saturation magnetic flux density Bs of the magnetic substance that constitutes the single-pole head. As the theoretical limit of the saturation magnetic flux density Bs of the magnetic substance is 3.0 tesla, which is merely 1.5 times the theoretical limits of materials available at present and cannot cope with the future higher density.
The conventional magnetic recording has been done by inverting the magnetization of a magnetic recording medium using the magnetic field generated by the induction type magnetic head which floats and runs over the magnetic recording medium. A recent magnetic random access memory (MRAM) which is a possible substitute for the conventional dynamic random access memory (DRAM) employs a recording system which inverts the magnetization of one of tunneling magnetoresistance effect (TMR) elements having a multilayered structure of a magnetic film/non-magnetic insulating film/magnetic film by using a combined magnetic field formed by the current that runs through two metal lines laid on and under this TMR element, perpendicular to each other (see, for example, Patent Document 1: U.S. Pat. No. 5,734,605). It is however pointed out that the MRAM has a shortcoming that as the size of TMR elements is made smaller for larger memory capacity, the size of the magnetic field needed to invert magnetization becomes larger, requiring that a lot of current should flow through the metal lines. This leads to an increase in power consumption and eventually damages the lines. Those facts suggest that the magnetic disk apparatus as well as the MRAM suffer limitation to increasing the density as long as the magnetization inversion system using a magnetic field is employed.
As a scheme of inverting magnetization without using a magnetic field, a magnetic recording system has been proposed which makes magnetic recording by causing the current to flow to a magnetic recording medium from a probe having a conductive chip and heating the portion where the current has run (see, for example, Patent Document 2: Japanese Patent Laid-Open No. Hei 5-206146).
There is an empirical report on a recording system which forms pillars of 130 nm in diameter each including a multilayered film of Co/Cu/Co between two Cu electrodes and inverts the magnetization of the Co layer by letting the current flow in the pillars (see, for example, Non-patent Document 2: Physical Review Letters, Vol. 84, No. 14, pp. 3149-3152 (2000)).
Several ideas of constructing a super high density writing/reading apparatus without using magnetic recording have also been proposed. One of the proposed ideas is a recording system which uses an array of 32×32 cantilevers 102 each having a heater formed at its distal end and polycarbonate and makes recording by heating the heater at the distal end of a cantilever and pressing the heater against the polycarbonate to deform the polycarbonate, thereby forming holes therein (see, for example, Non-patent Document 3: Applied Physics Letters, Vol. 77, No. 20, pp. 3299-3301 (2000)).
The conventional proposals however have the following problems.
In the system that makes magnetic recording by causing the current to flow to a magnetic recording medium from a probe and heating the current-flown portion, for example, magnetic recording is done by the Joule heat of the supplied current, so that the inversion of magnetization takes time. The system cannot therefore ensure as fast recording as achieved by the conventional magnetic disk apparatus and MRAM.
The recording system that forms pillars of 130 nm in diameter each including a multilayered film of Co/Cu/Co and inverts the magnetization of the Co layer by letting the current flow in the pillars requires a current density of about 3×107 (A/cm2) for recording. In case where such a large current is provided from the metal lines of the conventional MRAM, therefore, the current density that even lines of a tungsten-based material which has a large durability to the current can withstand is about 1×107 (A/cm2) so that the cross-sectional area of the lines cannot be made smaller than a 150-nm square. This stands in the way of increasing the density. In addition, because the system requires that a current of a high current density should run through long lines, the system has a reliability problem.
As the system that makes recording by heating the heater at the distal end of a cantilever and pressing the heater against polycarbonate to deform the polycarbonate and form holes therein also uses heat and suffers a slow writing speed of several tens of microseconds. While this prior art proposes a way of increasing the writing speed by parallel writing with a total of 1024 cantilevers, the writing speed of each cantilever is slow which, it seems, makes it very hard to achieve the fast recording done by the magnetic disk apparatus.